Because optical materials formed from synthetic resin are light compared to inorganic materials such as glasses, excellent in molding processability and the like, and easy to handle, such optical materials have been widely used in various applications in recent years. Polystyrene resin, polymethylmethacrylate resin, polycarbonate resin, diethylene glycol diallyl carbonate resin, and the like have been heretofore used as such resin for organic optical materials.
When these resins for organic optical materials are used as, for example, plastic lenses for glasses, the resins must have a high refractive index to reduce the lens thickness. Further, an attempt has been made to use these resins as light-transmission bodies by providing a refractive index distribution to transparent resin.
However, previous resins for organic optical materials are not always satisfactory because these resins have drawbacks such as a low refractive index, a high birefringence, a high dispersibility, and the like; and are also poor in heat resistance and shock resistance. In particular, diethylene glycol diallyl carbonate resin (CR-39) and the like used as lens materials have a low refractive index (1.50). Therefore, when these resins are used as lenses, the edge thickness and the central thickness become thick, causing drawbacks such as degradation in the appearance of the lenses and an increase in the weight.
Consequently, attempts have been made to improve the refractive index of resin for organic optical materials. For example, as a monomer for producing resin having a high refractive index and excellent transparency, Patent Literature 1 and Patent Literature 2 listed below disclose a diaryl sulfide compound represented by the following chemical formula (a):
wherein R1 to R4 represent hydrogen, C1-4 alkyl, or halogen, and R5 represents C2-6 alkenyl.
While these diaryl sulfide compounds are described as monomers for producing resin having a high refractive index and excellent transparency, the production of these compounds requires a 4,4′-dihalodiaryl sulfide compound, which is an expensive compound represented by the following chemical formula:
wherein R1 to R4 represents hydrogen, C1-4 alkyl, or halogen, and X represents halogen. Accordingly, the diaryl sulfide compound of the above chemical formula (a) obtained using the 4,4′-dihalodiaryl sulfide compound as a starting material is costly, and the economic efficiency thereof is low. Therefore, there is a demand for a less-expensive material as a monomer that can be used for producing resin having a high refractive index and excellent transparency.
Meanwhile, diphenyl sulfone, diphenyl sulfone derivatives such as 4,4′-dichlorodiphenyl sulfone and 3,3′-4,4′-tetrachlorodiphenyl sulfone, and sulfur-containing compounds such as diphenyl sulfide and diphenyl sulfoxide are known as additives used for changing the refractive index of resin for organic optical materials (see Patent Literature 3 and Patent Literature 4 listed below).
However, of the above-described additives, diphenyl sulfone, diphenyl sulfone derivatives, and the like have a low refractive index (about 1.6), and these additives must be added in a large amount in order to increase the refractive index of the resin. This may deteriorate the properties of the resin, and it is not economically desirable.
Diphenyl sulfide, diphenyl sulfoxide, and the like require an expensive substance as a starting material for synthesis. Therefore, these compounds are costly, and are not economically desirable, particularly when they are added in a large amount.